Popup component from the UI class is rendered transparent and without any border. Please run the following snippet to see the issue.
let nRowCount = 1000;
let nColCount = 100;
let ggrid = grok.shell.addTableView(grok.data.demo.randomWalk(nRowCount, nColCount)).grid;
let button = null;
let btn = ui.button('Click', function(){
let eContent = ui.splitV([ui.divText("AAAAAAAAAAAAAAAAAAAA"), ui.divText("BBBBBBBBBBBBBBBBBBB")]);
let popup = ui.showPopup(eContent, button, true);
});
button = btn;
ui.dialog("Popup Bug Demo").add(btn).show();